This invention relates generally to an imaging optical system such as a reflection and refraction optical system and, more particularly, to a reflection and refraction optical system usable for imaging a fine pattern in the manufacture of microdevices such as semiconductor devices (such as ICs or LSIs), image pickup devices (such as CCDs) or display devices (such as liquid crystal panels). In another aspect, the invention is concerned with a projection exposure apparatus using such a reflection and refraction optical system.
The degree of integration of a semiconductor device such as an IC or LSI is increasing, and the fine processing technology for a semiconductor wafer is being developed considerably. In the projection exposure technique which is the main aspect of the fine processing technology, the resolution has been increased to a level allowing formation of an image of a linewidth not greater than 0.5 micron.
The resolution can be improved by shortening the wavelength of light used for the exposure process. However, the shortening of the wavelength restricts the glass materials usable for a projection lens system, and correction of chromatic aberration becomes difficult to attain.
A projection optical system, with which the difficulty of correcting chromatic aberration can be reduced, may be a reflection and refraction optical system comprising a concave mirror and a lens group, wherein the imaging function mainly attributes to the power of the concave mirror.
Such a reflection and refraction optical system may include a polarization beam splitter, a quarter waveplate and a concave mirror, disposed in this order from the object plane side. Light from the object plane may go by way of the polarization beam splitter and the quarter waveplate, and it may be reflected by the concave mirror. After this, the light may go again by way of the quarter waveplate and the polarization beam splitter, and it may be imaged upon an image plane. The combination of the polarization beam splitter and quarter waveplate may be effective to reduce the loss of light. However, use of rectilinearly polarized light for the imaging process may involve a problem in the formation of a fine image of a linewidth not greater than 0.5 micron that the imaging performance may change in dependence upon the orientation (lengthwise direction) of a (linear) pattern on the object plane.
As an example, the contrast of an image of 0.2 micron, which can be formed by using a projection optical system of a numerical aperture (N.A.) of 0.5 and a design wavelength 248 nm together with a phase shift mask (line-and-space pattern), is changeable by about 20%, depending on whether the direction of polarization of light used for the imaging process is parallel to or perpendicular to the lengthwise direction of the pattern.
It is an object of the present invention to provide an improved imaging optical system effective to solve such a problem as described above.
It is another object of the present invention to provide an improved reflection and refraction optical system effective to solve such a problem as described above.
It is a further object of the present invention to provide an improved projection exposure apparatus which is free from such a problem described above.
An imaging optical system according to the present invention may include a polarization beam splitter, a quarter waveplate and a reflection mirror which may be disposed in this order from an object plane. Light from the object plane may go by way of the polarization beam splitter and the quarter waveplate, and it may be reflected by the reflection mirror. The reflected light may go again by way of the quarter waveplate and the polarization beam splitter, and then it may be imaged upon an image plane. Means may be provided between the polarization beam splitter and the image plane, for changing the plane of polarization of polarized light from the polarization beam splitter.
A reflection and refraction optical system according to the present invention may include a polarization beam splitter, a quarter waveplate and a concave reflection mirror which may be disposed in this order from an object plane. Light from the object plane may go by way of the polarization beam splitter and the quarter waveplate, and it may be reflected by the concave reflection mirror. The reflected light may go again by way of the quarter waveplate and the polarization beam splitter, and then it may be imaged upon an image plane. Means may be provided between the polarization beam splitter and the image plane, for changing the plane of polarization of polarized light from the polarization beam splitter.
A projection exposure apparatus according to the present invention may include a projection optical system for projecting a pattern of a mask onto a substrate to be exposed. The projection optical system may comprises a polarization beam splitter, a quarter waveplate and a concave reflection mirror which may be disposed in this order from the mask. Light from the mask may go by way of the polarization beam splitter and the quarter waveplate, and it may be reflected by the concave reflection mirror. The reflected light may go again by way of the quarter waveplate and the polarization beam splitter, and then it may be directed to the substrate such that the pattern of the mask may be imaged upon the substrate. Means may be provided between the polarization beam splitter and the image plane, for changing the plane of polarization of polarized light from the polarization beam splitter.
A reflection and refraction optical system or a projection exposure apparatus according to the present invention may suitably be used for the manufacture of microdevices such as semiconductor devices (such as ICs or LSIs), image pickup devices (such as CCDs) or display devices (such as liquid crystal panels). Particularly, a reflection optical system of the present invention, when arranged to provide a reduction magnification and used as a projection optical system in combination with deep ultraviolet light, may be effective to image a fine device pattern of a linewidth not greater than 0.5 micron.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.